1. Field of the Invention
The present invention relates to novel adducts useful for improving toughness, and curable compositions using such adducts. The novel adducts may improve toughness, such as in terms of impact resistance, and/or adhesion in curable compositions using those adducts.
2. Brief Description of Related Technology
Toughness generally is the ability of a material to absorb energy and undergo large permanent set without rupture. For many engineering adhesive applications, toughness is often the deciding factor. Plastics, because of their inherent brittleness, have heretofore been modified in a variety of ways in efforts to improve the toughness thereof. Epoxy resins, for example, which form a versatile glassy network when cured, exhibit excellent resistance to corrosion and solvents, good adhesion, reasonably high glass transition temperatures (Tg) and adequate electrical properties. Unfortunately, however, the poor fracture toughness of epoxy resins oftentimes limits the usefulness thereof in many commercial applications.
The impact strength, as well as other physical properties of crosslinked epoxy resins, is controlled by the chemical structure and molecular weight of the epoxy resin, weight ratio of the epoxy resin to the hardener, by any added fillers, and by the conditions used to cure the formulation. Unfortunately, crosslinked, glassy epoxy resins with a relatively high glass transition temperature (“Tg”) (>100° C.) are brittle in nature. The poor impact strength of high glass transition epoxy resins limits their usage as structural materials and use in or as composites.
Conventional toughening agents (e.g., carboxyl terminated butadiene nitrile rubbers, “CTBN”) are frequently unsuitable as additives in formulations where low temperature crash impact performance is desired.
Carbonyl biscaprolactam, such as is available commercially under the ALLINCO brand name from DSM Research, has been reported as a versatile, nontoxic reagent that converts hydroxy and amino groups of functional polymers into the corresponding caprolactam-blocked isocyanates without requiring the use of isocyanates. See Angew. Chem. Int. Ed., 42, 5094-5097 (2003). For instance, in the context of primary amines, see the following reaction scheme I:

In addition, U.S. Pat. No. 5,278,257 (Mulhaupt) and International Patent Publication No. WO 2005/007766 A1 describe the preparation of a rubber modified epoxy composition containing a phenol-capped polyurethane pre-polymer as a toughening agent. The so-described toughening agents are believed to be the basis of the BETAMATE-brand product offering from Dow Automotive.
The low temperature performance properties of such BETAMATE-brand products could stand improvement. In addition, consumers would benefit from the offering of additional adducts and products using such adducts having different or more desirable physical property performance.
Accordingly, there is a need for novel adducts that are effective for improving the toughness of adhesive formulations, especially in formulations requiring good low temperature performance, and which formulations are based on thermosets such as epoxy, episulfides and/or benzoxazines.